1. Field of the Invention
The present invention relates to switched reluctance machines, and relates particularly to switched reluctance machines with segmented rotors.
2. Description of Related Art
Switched reluctance electrical motors are known in which a stator having a series of circumferentially arranged stator poles of soft magnetic material has current-carrying coils wound on the poles. A rotor having rotor poles of magnetic material is rotatably arranged with respect to the stator and seeks to align itself with the magnetic field generated by the coil or coils which are energized. The coils of the stator poles are energized to develop torque on the rotor in the desired sense relative to the stator.
The amount of torque generated by such machines is dependent upon the difference between the flux linking each turn of the excitation coils in the aligned (i.e. rotor and stator poles aligned) condition and the non-aligned (i.e. maximum non-alignment of rotor and stator poles) conditions. The aligned and unaligned flux linkage rises as the width of the stator pole arc rises as a proportion of the pitch of the stator poles, but once the arc of the stator poles becomes more than half of the spacing between adjacent rotor poles, there is always overlap between rotor and stator poles, even in the unaligned condition, which causes the unaligned inductance to rise rapidly. As a result, the peak torque generated by such machines is found to arise when the ratio of stator pole width to stator pole pitch is about 0.42.
In an attempt to overcome this limitation, switched reluctance machines have been produced in which the rotor, instead of having radially extending poles, is provided with magnetically separated segments of magnetic material which direct magnetic flux between adjacent stator poles. In these machines, little flux crosses the rotor diametrically. Such machines are described in “The design of switched reluctance motors with segmental rotors” by Mecrow, B. C., Finch, J. W., El-Kharashi, E. A. & Jack, A. G. in the Proceedings of ICEM 2002, International Conference on Electrical Machines, 25–28 Aug. 2002, Brugge, Belgium, incorporated herein by reference.
For a simplified arrangement in which the arc of the stator pole is equal to that of the rotor segments, it is found that as the ratio of stator pole width to pole pitch rises above 0.5, the useful torque generated by the machine continues to rise. However, at the same time, as the ratio of pole arc to pole pitch becomes higher, the slot between adjacent stator poles becomes very narrow, with the result that insufficient space is available to accommodate the excitation winding around each pole.
Attempts to overcome this problem are disclosed in U.S. Pat. No. 5,010,267 to Lipo and European Application No. 0504093 to Horst, both of which are incorporated herein by reference, in which each current carrying winding has a pitch of more than one stator pole, such that the windings of alternate phases (i.e. windings which are not energized at the same time) occupy adjacent slots between stator poles. This gives the advantage that the magnetic flux generated by two sides of any winding is not directed along a single tooth, as a result of which magnetic saturation in that tooth is avoided. However, prior art motors of this type suffer from the drawback that the windings extending over more than one stator pole need to be connected by long end connections, which increases the overall winding loss and causes the motor to occupy more space. This limits the extent to which the motor can be made at low cost.